As of September of 2012, 8.7 million acres have burned in the western United States; 2.6 million acres were in the Great Basin alone. Much of the burned acres were federally managed by the Bureau of Land Management or Forest Service. Many permittees that graze livestock on these public lands will lose their ability to graze for at least the next two years, under the premise that bunchgrasses need rest from grazing to survive and recover after fire.

Rangelands that have been invaded by cheatgrass may have little perennial bunchgrass returning due to subsequent competition with annual invasive weeds. However, cheatgrass will continue to grow each year and increase fine fuel loads; which after two years could easily prime these burned and rested lands for another catastrophic wildfire.

Great Basin ranchers have already experienced this burn-rest phenomenon where allotments have not been grazed for several years, due to the mandates of the BLM or FS, and then burn in years subsequent to rest from grazing.

A new concept, building upon state-and-transition model theory is Disturbance Response Groups (DRGs). DRGs are groups of ecological sites that respond similarly to disturbance with varying rates of responses, but with the same end point. DRGs are important to land managers because they allow management decisions to be applied over a large area, simplifying the complex decision making process in the wake of large fire events.

The objective of this study is to measure plant community responses to no grazing and three different defoliation/grazing treatments after wildfire. By measuring the DRGs scale rather than the ecological site scale, this project will determine the utility and value of DRGs for land management decisions.

Additionally, this project will assess the plant community’s response for a variety of threshold values. Being able to identify threshold triggers will enable managers to make better pre- and post-disturbance decisions.

Understanding the role of grazing after fire in bunchgrass recovery and cheatgrass management will provide land managers with knowledge to promote post-fire recovery, fuels management and subsequently smaller and fewer fires.

Recent research by Bates (2009) indicated that properly applied grazing management does not impede recovery of perennial bunchgrasses and the herbaceous understory in prescription-burned Wyoming big sagebrush dominated communities. The study also reported no difference in plant cover, density, composition, annual yield, bare ground, and litter among grazed and ungrazed burn treatments. However, the study location had minimal pre-and post-fire annual grass presence, unlike many of the areas that recently burned from wildfires in the Great Basin. Likewise, Bruce et al. reported that livestock grazing had no effect on plant density or cover post-wildfire; however, this study also had minimal cheatgrass presence (<5% cover).

A recent study by Diamond et al. (2009) reported that cattle grazing reduced cheatgrass biomass and cover, which resulted in reduced flame lengths and rate of spread in cheatgrass invaded rangelands in northern Nevada. Similarly, Schmelzer (2009) reported significant reductions in cheatgrass biomass with fall cattle grazing in central Nevada. He also reported no change in perennial grass cover or density after three years of fall cattle grazing; suggesting that fall grazing was not detrimental to perennial bunchgrass survival.

Keeping cheatgrass production down may allow for better perennial grass recovery post-fire by reducing the threat of multiple catastrophic wildfires within a short time period. Additionally, reducing the threat of wildfires in annual dominated areas may reduce risk to adjoining relatively intact plant communities. In other words, the higher ignition probabilities of cheatgrass-dominated plant communities impact all plant communities on the landscape.

Most research indicates that fall or dormant season grazing post-fire has minimal or negligible impacts on perennial bunchgrass mortality and vigor. Bunting et al. (1998) reported that Idaho fescue and bluebunch wheatgrass exhibited low mortality when exposed to fire alone and 50 or 70% mortality when burning was followed by two seasons of early season defoliation. However, when Idaho fescue experienced two late season (early fall) defoliations after fire, plant mortality ranged from 0-10% and was less than burned plants without defoliation. Idaho fescue also had better reproductive culm production when burning was combined with late season defoliation compared to burning or burning with early season defoliation.

Similarly, Jirik and Bunting (1994) reported late season defoliation the first year post-fire did not reduce vigor of bluebunch wheatgrass or squirreltail, even though their study occurred with below average precipitation and “heavy use” (clipping to 2 cm stubble height).

Although no grazing treatment was applied, in Oregon a burning study showed squirreltail plants responded positively to prescribed summer burning compared to unburned plants one year post-fire (Young and Miller 1985). Burned plants had more shoot, crown, and root biomass, more inflorescence biomass, and higher shoot density per unit crown area (Young and Miller 1985). That study also reported no difference in cheatgrass production between burned and unburned treatments; however, cheatgrass density was significantly higher in the unburned treatment.

Davies et al. (2009) reported less than 2% cover of cheatgrass in grazed-burned treatments whereas ungrazed-burned treatments had about 6% cover in a northern Great Basin study. These results suggest that grazing before fire may reduce the occurrence of cheatgrass post-burn. Additionally, prior disturbances can influence the response of plant communities to subsequent disturbance (Davies et al. 2009). Therefore, grazing before fire may decrease the cheatgrass response post-fire, while grazing after fire may further hinder cheatgrass response while reducing the risk of another fire within a short time period (i.e., burn-rest-burn).

Cheatgrass Competition

A 60-day greenhouse study conducted by Svejcar (1990), showed cheatgrass had twice the root length and leaf area of crested wheatgrass. These results suggest why cheatgrass is more competitive than surrounding bunchgrasses and exploitive of the soil moisture and nutrient resources that are limiting in our arid environments. West and Yorks (2002) data suggested that annual grass cover varies depending on precipitation, independent of grazing and burning treatments; with wet years having more annual grass cover than drought years. Thus a multi-year project is critical to the overall understanding of ecosystem dynamics after fire.

State-and-Transition Model Theory

The importance of understanding post-fire disturbance response is critical to management of these dynamic rangelands. The use of state-and-transition models (STM) has been accepted by the Natural Resource Conservation Service (NRCS) for describing plant community development and ecological dynamics for ecological sites (USDA 1997), as well as associated dynamic soil properties.

Ecological resilience is defined as the amount of change or disruption that is required to transform a system from being maintained by one set of mutually reinforcing processes and structures to a different set of processes and structures. Thus, resilience in the STM concept is how far a system can be displaced before the return to equilibrium is precluded (Stringham et al. 2003), or how resilient a state is to disturbance.

Thresholds are defined as a boundary in space and time between states, such that one or more of the primary ecological processes has been irreversibly changed and must be actively restored before return to a previous state is possible (Stringham et al. 2003). Another way to think of this is that a threshold represents a point in time and space at which the primary ecological processes (e.g., infiltration, energy capture) degrade beyond the ability to self-repair (Petersen et al. 2009). Thresholds occur when conditions are sufficient to modify ecosystem structure and function beyond the limits of ecological resilience (Briske et al. 2008).

Since STMs and thresholds are process based, the ecological processes of an ecological site must be measured to determine if a state change or threshold event has occurred. Ecological processes include hydrologic function, nutrient cycling, and energy capture. Aggregate stability, infiltration rate, annual production, basal gap and foliar cover are all measurable surrogates for ecological function and process. These measurements can then be analyzed to assess thresholds within STMs. Several studies have used multivariate statistical analysis methods to analyze community data and define threshold values (Carr 2007; Petersen et al. 2009; Matney 2010).

A “trigger” is an event that initiates a threshold-related process due to the loss of resilience (Briske et al. 2008), which results in a state change. Therefore, catastrophic wildfires like those experienced in the 2012 fire season may have been triggers for undesired state changes on many sites across the Great Basin. Consequently, many areas have crossed an ecological threshold and will require active management and likely a large economic input (e.g. seeding, herbicide treatments, etc.) to return to a more desired state.

Understanding where thresholds have been crossed allows a triage approach to restoration and increases the knowledge foundation for post-disturbance management decisions. Understanding plant community response to disturbance at large management scales is made more difficult by the inherent variability of rangelands across large landscapes. The Great Basin is a vast and dynamic complex of ecological sites.

Disturbance response groups (DRGs) bridge the gap between ecological sites, which are smaller than most practical land management units, and Major Land Resource Areas (MLRA) which are much larger and made up of associated land resource units (USDA 1997). DRGs occur within an individual MLRA, with each DRG having one to several ecological sites. Some ecological sites are so unique that their disturbance response and ecological dynamics are unlike any other site, thus a DRG would have only one ecological site. However, some ecological sites have similar plant, soil, climate, and landform characteristics making disturbance response similar and enabling grouping into a larger unit, and an easier scale to utilize for land management decisions.

This research will build upon prior burn-graze research but will also incorporate multi-year response across landscape scale DRGs. In addition, this project will assess community response to fire and defoliation, not just single species response.

The overarching goal of this landscape-scale project is to expand our understanding of the response of annual-dominated and annual invaded rangelands to post-fire grazing decisions. This will allow for improved science-based pre- and post-fire land management to occur at the disturbance response group scale. Objectives in support of this goal include:

Determine herbaceous community and dynamic soil property response to grazing post-wildfire under different combinations of season of defoliation and rest from defoliation.

Determine annual production and fine fuel loads post-wildfire under different combinations of season of defoliation and rest from defoliation.

Within disturbance response groups, examine community and environmental data to identify quantitative threshold values for the disturbance response group scale state-and-transition model.

Determine if post-fire grazing impacts on plant communities vary between disturbance response groups within a Major Land Resource Area.

Determine if post-fire grazing impacts on plant communities vary between similar disturbance response groups across Major Land Resource Areas.

Bunting, S., R. Robberecht, and G. Defosse. 1998. Length and timing of grazing on postburn productivity of two bunchgrasses in an Idaho Experimental Range. International Journal of Wildland Fire 8:15-20.